about_Language_Modes

מאמר
10/04/2023

Short description

Explains language modes and their effect on PowerShell sessions.

Long description

The language mode of a PowerShell session determines which elements of the PowerShell language can be used in the session.

PowerShell supports the following language modes:

FullLanguage
RestrictedLanguage
ConstrainedLanguage (introduced in PowerShell 3.0)
NoLanguage

What is a language mode?

The language mode determines the language elements that are permitted in the session.

The language mode is a property of the session configuration (or "endpoint") that's used to create the session. All sessions that use a particular session configuration have the language mode of the session configuration.

All PowerShell sessions have a language mode. Sessions are created using the session configurations on the target computer. The language mode set in the session configuration determines the language mode of the session. To specify the session configuration of a PSSession, use the ConfigurationName parameter of cmdlets that create a session.

Beginning in PowerShell 7.3, you can run pwsh with the ConfigurationFile parameter. This allows you to start PowerShell using a specific configuration.

Finding the language mode of a session

You can find the language mode of a FullLanguage or ConstrainedLanguage session by getting the value of the LanguageMode property of the session state.

For example:

$ExecutionContext.SessionState.LanguageMode
ConstrainedLanguage

However, in sessions with RestrictedLanguage and NoLanguage modes, you can't use the member-access operator (.) to get property values. Instead, the error message reveals the language mode.

When you run the $ExecutionContext.SessionState.LanguageMode command in a RestrictedLanguage session, PowerShell returns the PropertyReferenceNotSupportedInDataSection and VariableReferenceNotSupportedInDataSection error messages.

PropertyReferenceNotSupportedInDataSection: Property references aren't allowed in restricted language mode or a Data section.
VariableReferenceNotSupportedInDataSection: A variable that can't be referenced in restricted language mode or a Data section is being referenced.

When you run the $ExecutionContext.SessionState.LanguageMode command in a NoLanguage session, PowerShell returns the ScriptsNotAllowed error message.

ScriptsNotAllowed: The syntax isn't supported by this runspace. This might be because it's in no-language mode.

Finding the language mode of a session configuration

When a session configuration is created using a session configuration file, the session configuration has a LanguageMode property. You can find the language mode by getting the value of the LanguageMode property.

(Get-PSSessionConfiguration -Name Test).LanguageMode
FullLanguage

On other session configurations, you can find the language mode indirectly by finding the language mode of a session that's created using the session configuration.

Setting the language mode

The language mode in a PowerShell session can be set through the built-in $ExecutionContext variable.

$ExecutionContext.SessionState.LanguageMode = "ConstrainedLanguage"

However, doing this is only useful for experimenting with language modes. Language modes are intended to provide added security to PowerShell sessions for specific contexts.

Languages modes are set when you use a system application control policy or create a session configuration.

Using a system application control policy

PowerShell automatically runs in ConstrainedLanguage mode when it's running under a system application control policy. The application control policies detected are AppLocker and Windows Defender Application Control (WDAC) on Windows platforms.

PowerShell applies other restrictions besides language modes when it detects an application control policy. For example, there are additional restrictions to dot-sourcing and module importing under a policy.

When a PowerShell session is started under a policy, it runs in ConstrainedLanguage mode. Beginning in PowerShell 7.4, the start up banner display includes a message indicating it's running in that mode. This allows users to have a usable interactive shell experience, running cmdlets and native commands, as well as access to basic language elements. But the user can't access PowerShell, .NET, or COM APIs that could be abused by a malicious actor.

Any script or script-based module executed in this session runs in ConstrainedLanguage mode. However, any script or script-based module allowed by the policy runs in FullLanguage mode without any constraints. This way, a system locked down by policy can have scripts that are trusted by the policy and run with few constraints.

Using a session configuration

PowerShell remoting optionally supports creating custom session configurations. You can set the language mode you want for that custom configuration. PowerShell Just Enough Administration (JEA) configurations use NoLanguage mode to restrict sessions to command invocations only. With JEA, the remote session can be restricted to specific users. The JEA users are limited to running a defined set of commands and can't directly access APIs, the file system, or other system resources.

For more information, see JEA Session configurations and New-PSSessionConfigurationFile.

Language mode features and limitations

This section describes the language modes in PowerShell sessions.

FullLanguage mode

The FullLanguage mode permits all language elements in the session. FullLanguage is the default language mode for default sessions on all versions of Windows.

RestrictedLanguage mode

In RestrictedLanguage mode, users can run commands (cmdlets, functions, CIM commands, and workflows), but can't use script blocks. This mode is also used to process modules manifests loaded by Import-Module.

Beginning in PowerShell 7.2, the New-Object cmdlet is disabled in RestrictedLanguage mode when system lockdown is configured.

By default, only the following variables are permitted in RestrictedLanguage mode:

$PSCulture
$PSUICulture
$True
$False
$Null

Module manifests are loaded in RestrictedLanguage mode and may use these additional variables:

$PSScriptRoot
$PSEdition
$EnabledExperimentalFeatures
Any environment variables, like $ENV:TEMP

Only the following comparison operators are permitted:

-eq (equal)
-gt (greater-than)
-lt (less-than)

Assignment statements, property references, and method calls aren't permitted.

ConstrainedLanguage mode

ConstrainedLanguage mode is designed to allow basic language elements such as loops, conditionals, string expansion, and access to object properties. The restrictions prevent operations that could be abused by a malicious actor.

The ConstrainedLanguage mode permits all cmdlets and a subset of PowerShell language elements, but limits the object types that can be used.

The features of ConstrainedLanguage mode are as follows:

All cmdlets in Windows modules are fully functional and have complete access to system resources, except as noted.
All elements of the PowerShell scripting language are permitted.
All modules included in Windows can be imported and all commands that the modules export run in the session.
The Add-Type cmdlet can load signed assemblies, but it can't load arbitrary C# code or Win32 APIs.
The New-Object cmdlet can be used only on allowed types (listed below).
Only allowed types can be used in PowerShell. Other types aren't permitted. Type conversion is permitted, but only when the result is an allowed type.
Cmdlet parameters that convert string input to types work only when the resulting type is an allowed type.
The ToString() method and the .NET methods of allowed types can be invoked.
Users can get all properties of allowed types. Users can set the values of properties only on allowed types.

The following .NET types are permitted in ConstrainedLanguage mode. Users can get properties, invoke methods, and convert objects to these types.

Allowed Types:

[adsi]
[adsisearcher]
[Alias]
[AllowEmptyCollection]
[AllowEmptyString]
[AllowNull]
[ArgumentCompleter]
[ArgumentCompletions]
[array]
[bigint]
[bool]
[byte]
[char]
[cimclass]
[cimconverter]
[ciminstance]
[CimSession]
[cimtype]
[CmdletBinding]
[cultureinfo]
[datetime]
[decimal]
[double]
[DscLocalConfigurationManager]
[DscProperty]
[DscResource]
[ExperimentAction]
[Experimental]
[ExperimentalFeature]
[float]
[guid]
[hashtable]
[int]
[int16]
[int32]
[int64]
[ipaddress]
[IPEndpoint]
[long]
[mailaddress]
[Microsoft.PowerShell.Commands.ModuleSpecification]
[NoRunspaceAffinity]
[NullString]
[Object[]]
[ObjectSecurity]
[ordered]
[OutputType]
[Parameter]
[PhysicalAddress]
[pscredential]
[pscustomobject]
[PSDefaultValue]
[pslistmodifier]
[psobject]
[psprimitivedictionary]
[PSTypeNameAttribute]
[ref]
[regex]
[sbyte]
[securestring]
[semver]
[short]
[single]
[string]
[SupportsWildcards]
[switch]
[timespan]
[uint]
[uint16]
[uint32]
[uint64]
[ulong]
[uri]
[ushort]
[ValidateCount]
[ValidateDrive]
[ValidateLength]
[ValidateNotNull]
[ValidateNotNullOrEmpty]
[ValidateNotNullOrWhiteSpace]
[ValidatePattern]
[ValidateRange]
[ValidateScript]
[ValidateSet]
[ValidateTrustedData]
[ValidateUserDrive]
[version]
[void]
[WildcardPattern]
[wmi]
[wmiclass]
[wmisearcher]
[X500DistinguishedName]
[X509Certificate]
[xml]

Only the following COM object types are permitted:

Scripting.Dictionary
Scripting.FileSystemObject
VBScript.RegExp

NoLanguage mode

PowerShell NoLanguage mode disables PowerShell scripting language completely. You can't run scripts or use variables. You can only run native commands and cmdlets.

Beginning in PowerShell 7.2, the New-Object cmdlet is disabled in NoLanguage mode when system lockdown is configured.

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